The Diameter of Pipes and Plastic Tubes: Double is More Than Double

The rule of thumb in pipe sizing is twice the diameter equals four times the flow. Two one-inch pipes do not equal a two-inch pipe. It actually takes four one-inch pipes to carry the same load as a single two-inch pipe. Therefore, if you split a two-inch pipe into two one-inch pipes, you are cutting its flow capacity in half.

People who plumb with large pipes are usually aware of this, but with smaller tubing it’s easy to overlook this basic law of nature.

With the small flexible plastic tubing used for undersink filters and reverse osmosis units it is important to know that what is called 1/4″ tubing is measured by its outside diameter. With 1/4″ tubing, the inside diameter, the path that the water flows through is actually about 1/8″. What we call 3/8″ tubing is roughly 1/4″ inside diameter, so following the rule of thumb of pipe flow, a 3/8″ OD tube will actually carry four times the water flow as compared with the 1/4″ OD tube. This is a very important fact to keep in mind if you’re planning to send water from your RO unit across the room to an icemaker or refrigerator. Especially if your run is long, you’ll have a much better result with 3/8″ tubing than with 1/4″.